The present invention relates to a toner used for formation of images by means of electrophotography. Also, the present invention relates to a method of image formation in which the toner is used.
In the image formation of electrophotography, toner is fixed onto an image support by the method of heat roller fixing, so that a permanent visible copied image can be provided. When a transparency sheet for use in an overhead projector, which will be referred to as an "OHP sheet" hereinafter, is used as an image support and a copied color toner image is formed on an OHP sheet, it is necessary, particularly when color toner is used, to make the image surface smooth so as to enhance the optical transmittance in the case of projecting the image by an overhead projector. That is, it is required to fix the toner image smoothly on the OHP sheet surface so as to prevent the scatter and irregular reflection of transmitted light on the image surface.
In order to accomplish the above object, conventionally, a color toner which is capable of sharply transiting to a molten state of a visco-elasticity lower than that of ordinary black toner at the milting point thereof is used, and the surface of a formed color toner image is easily made to be smooth when the image is heated and pressed by a heat roller for fixation.
However, when the visco-elasticity of toner is lowered, the glass transition point of the toner is also lowered. Therefore, at a normal temperature or a temperature in an octual machine which the toner is used, the mechanical strength of the toner is deteriorated. Accordingly, when a mechanical force is impressed upon the toner particle by agitating in the developing apparatus, additive on the surface of the toner particle is embedded in the toner particle, and the performance of development and transfer of it is deteriorated. Further, adhesion of toner particles to carrier particles occurs. These problems are remarkably caused when the sizes of toner particles are reduced to enhance the image quality recently. The reason is that the smaller the toner particle size becomes, the more the toner particle is subjected to mechanical stress.
When the color toner with a low visco-elasticity is used, other problems are caused, which will be described below. Even when a releasing agent is added to toner so as to improve the releasing property in the fixing process, the viscosity of toner and that of the releasing agent are close to each other, so that the releasing agent does not function substantially. For this reason, in order to prevent the occurrence of hot offset in the fixing process, it is necessary to coat the surface of a fixing roller with silicon oil. As a result, in a copie image on OHP sheets support, a sufficiently high transmittance of image switable for projection by OHP hardly be obtaine because sticky silicon oil is coated unevenly on the surface. Further, it is necessary to enhance the wettability of silicon oil on the surface of a fixing roller. Therefore, it is substantially impossible to provide a coating layer of high releasing property made of fluororesin on the fixing roller. Accordingly, it is inevitable to use silicon rubber to make the fixing roller. As a result, the durability of the fixing roller is very low.
The present invention has been achieved to solve the above problems. It is an object of the present invention to provide a toner by which a smooth fixed copied image can be formed on an OHP sheet, so that a sufficiently high transmittance is maintained; and the mechanical strength of toner particles is sufficiently high and additive is not embedded in the toner particles even in the case of toner particles of small size.
It is another object of the present invention to provide a technique of forming a copied image, when a releasing agent is added to toner, it is possible to prevent the occurrence of hot offset in the fixing process and it is not necessary to coat silicon oil on the surface of a fixing roller, so that a copied image can be formed on an OHP sheet on which sticky silicon oil is not provided and there is no oil coated unevenly.
It is still another object of the present invention to provide a technique in which it is substantially unnecessary to coat silicon oil on the surface of a fixing roller; and it is possible to provide a covering layer of high releasing property made of fluororesin on the surface of the fixing roller, so that the durability of the fixing roller can be improved.